To determine the functional relationship between the MC29 (avian mycelocytomastosis virus) onc gene and its cellular prototypes, we have determined the sequence of the chicken and human c-myc genes and compared them to the MC29 transforming gen (v-myc). Both cellular genes contain sequences homologous to v-myc in two major exons and interrupted by a single intron with no viral homology. While the total number of viral-specific nucleotides in chicken c-myc is identical to the number in v-myc, the human c-myc has additional nucleotides, concentrated in the 5 feet exon. The two exons of the cellular genes and the viral gene have a common reading frame and protein terminator signal. The protein predicted from the reading frame suggests myc encodes a conserved gene product. The homology seen between the cellular genes is conserved only in the coding region. Both cellular genes differ from the v-myc by additional 5 feet sequences, substituted by the Deltagag of MC29 in myc protein. This may be the most significant difference between the cellular genes and the viral gene, perhaps eliciting functional differences between the gene products.